Prometheus, a Titan bound to a rock by Zeus, endured the daily torture of an eagle feasting on his liver, only to have the organ regrow each night. Compared with this spectacle, a video on the website of Nature this week seems decidedly dull. It shows a collection of pink dots consolidating into a darker central glob.

But something titanic is indeed happening. The pink dots are stem cells, and the video shows the development of a liver bud, something which can go on to look and act like a liver. Takanori Takebe and Hideki Taniguchi of Yokohama City University, in Japan, who made the video, have created working human-liver tissue.

Researchers have long dreamed that stem cells might be used to repair or replace damaged tissue, an aspiration known as regenerative medicine. Embryonic stem cells, in particular, are ‘pluripotent